Carbon nanotubes (CNTs) possess unique electrical and physical properties that make them desirable in many industrial applications. CNTs exist in two basic forms, single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). As the name suggests, SWCNTs are formed from a single layer of graphene, whereas MWCNTs are formed from multiple concentric layers of graphene. The physical structure of the SWCNT renders it superior to its MWCNT counterpart in its electrical and conductive properties. This feature gives the SWCNT special importance in electronic and semiconductor applications.
SWCNTs can exist in one of three structural types, “zigzag,” “armchair,” or “chiral,” depending on how the graphene sheet is “rolled” to form the cylindrical-shaped carbon nanotube. How the different types are formed may be understood by envisioning a single graphene sheet having a vector (n,m) defined by the integers “n” and “m” along which the sheet is rolled to form the tube, wherein “n” and “m” describe the diameter and chirality of the nanotube. If n=m, the carbon nanotube structure is referred to as an “armchair” and is characterized by its metallic properties. If m=0, the carbon nanotube is said to have a “zigzag” structure. Semiconducting carbon nanotubes have many applications including forming a thin film semiconducting coating on a substrate such as a transistor.
Conventional methods of producing single-walled carbon nanotubes (SWCNTs) include plasma arcing, carbon arc discharge, dual-pulsed laser vaporization techniques, and chemical vapor deposition. These methods typically yield a mixture of carbon nanotubes having metallic properties (m-SWCNTs) and carbon nanotubes having semiconducting properties (s-SWCNTs). For most modern applications, however, only SWCNTs with semiconducting properties are desirable. Thus, before the semi-conducting SWCNTs can be utilized they must first be separated from the metallic SWCNTs.
There exists, therefore, a need for effective and efficient separation of the metallic SWCNTs from the semiconducting SWCNTs on a bulk scale in order to obtain a purified batch of semiconducting SWCNTs. Known separation techniques include methods that rely on the innate subtle differences in density between the m-SWCNTs and the s-SWCNTs. These methods do not result in a quantitative separation and are not scalable to large quantities. Other methods rely on chemoselective electron transfer reactions that react more quickly with metallic carbon nanotubes. These methods are limited in that functionality has not been introduced to effectively separate the reacted (metallic) from the unreacted (semiconducting) CNTs.
More recently, Japanese Patent Publication No. 2007031238 (Feb. 8, 2007) discloses a method of separating metallic carbon nanotubes from semiconducting carbon nanotubes. The method comprises the steps of reacting the sodium salt of para-amino phenol with nitrosonium boron tetrafluoride to obtain a diazonium salt. The diazonium salt is then slowly added to a mixture of carbon nanotubes that have been previously dispersed in a liquid. The mixture contains both metallic and semiconducting nanotubes. The metallic nanotubes preferentially react with the diazonium salt due to higher electron density on the surface of the metallic nanotubes. The functionalized metallic nanotubes, containing as pendant groups the sodium salt of phenol, are then reacted with a functionalized particle, e.g., a particle containing halide groups on the surface thereof. The functionalized nanotubes react with the functionalized particles forming metallic carbon nanotubes coupled with particles. The metallic carbon nanotubes coupled with particles are readily removed from solution via a physical separation.
Yukata Maeda et al. disclose in the Journal of the American Chemical Society, 127(29), 10287-10290 (2005), a dispersion-centrifugation process in a THF solution of amine. The process allows a high concentration (up to 87%) of metallic SWCNTs to be obtained.
Kay Hyeok An et al. disclose in the Journal of Electronic Materials, 35(2), 235-242 (2006), a method of separating metallic single-walled carbon nanotubes from semiconducting single-walled carbon nanotubes. The method includes the steps of dispersing a powdered mixture of nanotubes in a solution of tetramethylene sulfone and chloroform, and including well dissolved nitronium ions. The dispersion is assisted by sonication. Nitronium ions are intercalated into nanotube bundles. The nitronium ions selectively attack the sidewall of the metallic SWCNT's due to the abundant presence of electron density at the Fermi level. After filtration, the semiconducting SWCNT's are left on the filter. The metallic SWCNT's are destroyed by the nitronium ions and drained away as amorphous carbons.